These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
That One Level: Sunset Vista. Clearing it is fairly challenging. Getting the gem, much more so. Remember: To get the Gem, one must destroy every crate and not die after reaching a Checkpoint Crate. Also, this is one of only two levels with a Cortex bonus level, which are hard and mandatory for 100%.
Slippery Climb had another level called "Stormy Ascent" in its style that was Dummied Outbecause it was too difficult.
Toxic Waste. You must avoid barrels that are thrown down the path by the other members of Pinstripe's mafia, and there are very few areas where you can hide from the barrels. It gets worse later on in the level, in which bouncing barrels are being thrown as well, and it takes lots of practice to memorize their patterns and where you can hide from them without being flattened.